This invention relates to gas sensors with a capability for self-testing.
Conventional electrochemical gas sensors operate by oxidising the gas at a sensing electrode, thereby generating a current. The rate of access to the electrode is determined by a diffusion barrier, and the rate at which the electrode can oxidise the gas is arranged to be very much greater than the rate at which gas diffuses through the barrier. Therefore the rate of oxidation, and hence the current, is controlled solely by diffusion, and this is a known value (for a given gas concentration) when the sensor is manufactured. If the activity of the electrode falls with time e.g. through poisoning, then the current eventually becomes limited by the lowered oxidation rate at the electrode and the sensitivity of the sensor falls. The sensor does not fail safexe2x80x94there is no way of telling from the cell output whether the gas concentration is low, or the concentration is higher and the electrode has lost activity.
Reliability of such sensors is ascertained by regular tests involving exposure to a calibration gas. In many situations, for example in a domestic CO safety monitor, this is undesirable. A sensor with a self test function, either triggered remotely or locally, would be highly advantageous.
GB-A-1,552,538 describes a self-test sensor assembly consisting of two parts, a sensor and a gas generation means, for example an electrolysis cell, joined by a delivery channel. Test gas is delivered directly to the sensing electrode of the sensor, with a membrane between the point of gas delivery and the outside world. Delivery is by a piston, a pressure difference resulting from the generation of gas itself, or other means. Signal gas enters the sensor from the atmosphere via the membrane. In this arrangement the concentration of test gas seen by the sensing electrode depends on the balance of the rate of generation of the gas and the rate of loss through the membranexe2x80x94the latter depends on the conditions (air flow) outside the membrane. As the generator is remote from the sensing electrode, there is a large volume to be filled with gas in order to ensure that a consistent known concentration is reached. This means the design is likely to require significant power, which is a limitation of the use of such a principle in a low power domestic monitor circuit.
GB-A-2245711 (corresponding U.S. Pat. No. 5,273,640 Kusanagi et al) describes a gas sensor with solid electrolyte layers disposed on two sets of electrodes, one designed for a gas sensing function, and the other set provided for a test function. The test function electrodes are arranged to sense a gas normally present in the atmosphere, e.g. oxygen. A decrease in the signal from the test electrode is taken to indicate a either a decrease in activity of the test electrodes, or a decrease in the permeability of the solid electrolyte, through which test and signal gas must pass before they reach the electrodes. Such change in permeability is a major factor in the performance of the sensor type disclosed in GB-A-2245711. The test of electrode decay rests on the assumption that the test electrodes will decay in the same way as the sensing electrodes. The test reaction using O2 is fundamentally different from the sensing reaction for oxidisable gases, being a reduction rather than an oxidation reaction, and so this form of test is likely to prove unreliable. A test where the sensing electrode oxidises test gas generated in known quantity, as in GB-A-1,552,538 would be advantageous.
The present invention provides a gas sensor including a housing containing at least a sensing electrode, a counter electrode, a test electrode, and electrolyte means in contact with such electrodes, the housing permitting gas from the environment to flow to the sensing electrode, and the gas sensor being such as to be operable either in a normal mode of operation in which potentials are applied to the electrodes for detecting when a gas to be sensed is present at the sensing electrode, or in a test mode of operation in which potentials are applied to the electrodes so that the test electrode generates a gas which flows to the sensing electrode to enable an indication whether the sensor is operating correctly.
Thus in accordance with the invention a cheap and accurate means is provided of self-testing, wherein the test gas is generated internally of the sensor and in a controlled amount by application of a suitable voltage potential.
A gas sensor according to claim 1 comprising of a planar arrangement of one or more sensing electrodes and one or more electrolytic generation electrodes on a common substrate in contact with common or separate electrolytes with associated counter and reference electrodes as may be required, such that the generation electrodes are close to the sensing electrodes, so as to minimise the amount of gas that is needed to effect the test. The gas might be delivered to the sensing electrode in the gas phase, by evolution into a communicating space above the electrodes, and access from generating to sensing electrodes might be via a diffusion barrier. The gas might alternatively be delivered to the sensing electrode in solution. The latter will give a measure of electrode activity different from, but related to, the activity measured for gas phase reaction, but will still give an indication of performance.
In a preferred embodiment, the planar arrangement of generating and sensing electrodes gives close proximity and small generated volumexe2x80x94hence low power and fast response. More than one generating electrode may be placed around the sensing or sensing electrode to further improve fast response and further reduce power requirements. An interleaved array of generating and sensing electrodes may also be employed. As preferred, screen printed electrodes and assembly method as described in our copending application WO 96/14576 (ref. PQ 12,622) is employed, that is: providing electrodes as porous planar elements on a substrate, a housing containing an electrolyte reservoir, and electrical terminals; positioning the substrate relative to the housing so that a portion of an electrode is positioned adjacent an electrical terminal; and bonding the substrate to the housing so that the electrode is electrically connected with the electrical terminal means while the porosity of the electrode is blocked in the region of the electrical connection to prevent permeation of electrolyte to the electrical connection. The electrodes are preferably formed of a porous electrically conductive material containing PTFE or similar polymeric binder, preferably particles of catalyst, and optional additional catalyst support material and material to enhance conductivity. The electrodes might be deposited onto the substrate by for example screen printing, filtering in selected areas from a suspension placed onto the substrate, by spray coating, or any other method suitable for producing a patterned deposition of solid material. Deposition might be of a single material or of more than one material sequentially in layers, so as for example to vary the properties of the electrode material through its thickness or to add a second layer of increased electrical conductivity above or below the layer which is the main site of gas reaction. The preferred metal deposit is platinum or platinum; carbon, although other deposits may be employed such as carbon or ruthenium dioxide.
The generator electrode may be placed close to the diffusion barrier inlet for signal gas, so that in self-test, some gas is lost to the outside and some is oxidised by the sensing electrode. If the diffusion barrier becomes blocked, the concentration seen by the sensing electrode during self-test is higher than would be the case without blockage, thus providing a means of checking whether the diffusion barrier is blocked. The accuracy of this check can be improved by delivering the test gas between two diffusion barriers.
Two levels of test may be provided: (1) a quick check of sensor function by generating gas in solution, which then diffuses to the sensing electrode through the solutionxe2x80x94this uses low power; and (2) a check on diffusion barrier blockage, which might also give a calibration of the sensor, in which gas is delivered to the sensing electrode in the gas phase as above. The cell may be provided with two generating electrodesxe2x80x94a submerged electrode without access to the gas phase for the first test, and an electrode on a porous substrate communicating with the gas phase for the second.
An actuator may be incorporated into the cell to close the diffusion barrier during self test. This would remove the effect of air currents on the test result. Comparison of open and closed responses test for blockage of the barrierxe2x80x94if there is no blockage, the closed response will be greater than the open response.